In a cochlear implant (CI) the amplitude of the acoustic audio signal has to be mapped to a relatively small dynamic range which can be delivered to the acoustic nerve. Typically two stages perform this amplitude compression, a front-end automatic gain control (AGC) which controls the overall loudness and an instantaneous non-linear mapping function of typically logarithmic shape which further compresses each band-pass envelope. The dynamic AGC used in current CI systems usually applies one gain to the entire analyzed frequency range before splitting the acoustic audio signal into individual frequency bands. Such systems have been shown to increase listening comfort and speech understanding in hearing aid (HA) users as well as CI users.
One possible drawback of such a system can occur in the presence of two signals which are located in different frequency regions, such as a speech signal in the presence of a continuous high frequency noise. In such an acoustic environment, the AGC gain would depend on the relative amplitudes of the two signals. Assuming a loud high frequency noise, the AGC gain would be reduced by the noise signal which could result in suppression of the speech signal. In a unilaterally implanted patient, this might only result in reduced speech understanding. But in bilaterally implanted patients, there could also be a reduced ability to localize sound sources. For example, assuming the loud high frequency noise source is located at the right side of the CI user, then the right-side AGC would reduce its gain more than the left-side AGC. Consequently, the interaural level differences at high frequencies would be reduced, and since the acoustic head shadow effect is higher at high frequencies, the interaural level difference at low frequency could vanish or even be inverted. As a result, the low frequency components, for example originating from a car engine, would be perceived from the wrong side.
These undesirable side effects of front-end single channel signal compression could generally be circumvented by using AGCs which compress individual band pass signals instead of the broadband signal. Such solutions can be found both in hearing aids and in cochlear implant systems. Speech understanding in the presence of noise sounds and sound localization in bilateral patients could potentially be enhanced. One major drawback of such systems is the fact that spectral differences such as amplitude differences in adjacent analysis bands get reduced. Spectral information such as formant frequencies in speech signals could also be less accessible to HA and CI users.
There have been previous efforts to apply dynamic compression to band pass signals. For example, FIG. 1 shows an arrangement described in U.S. Pat. No. 7,136,706 (incorporated herein by reference) which applies an overall mapping to a pre-band pass signal and then band specific mapping. The pre-band pass mapping function is thought to be linear (i.e. a limiter). The post-band pass mapping function is implemented as a non-linear, compressive, or logarithmic transform. The inventors state that the differences in acoustic spectrum component amplitudes are maintained. By maintaining these differences, spectral smearing between channels is reduced and speech cues are preserved. But dynamic adaptation of post-band pass compression would result in unwanted spectral smearing.
A second method which applies frequency specific gains is described in U.S. Pat. No. 6,731,767 (incorporated herein by reference). As shown in the block diagram in FIG. 2, an acoustic audio signal is split into a number of separate frequency bands and variable gain is applied to each frequency band independently. In contrast to previously used AGC circuits, the gain is controlled by a gain comparator and statistical estimates of each band pass signal are calculated and compared to predetermined hearing response parameters. Although the gain calculation appears to be dynamic in this patent, it does not describe any interaction between analysis channels.
U.S. Pat. No. 7,305,100 describes a dynamic compression process which applies channel specific gains for use in a hearing aid, although no mention is found of use in a cochlear implant system. Although as shown in FIG. 3, there is a gain control unit, no details are discussed with regards to interactions between the analysis frequency bands.
U.S. Patent Publication 2004/0136545 describes an arrangement for distributed gain control which takes into account the interactions between analysis channels. FIG. 4 shows a block diagram of the arrangement discussed which is described as providing a spectral enhancement system that includes distributed filters, energy distribution units, and a weighted-averaging unit. Instead of filter banks as used in cochlear implants and hearing aids, a filter cascade is used with an energy-detector that is coupled to each filter and provides an energy-detection output signal. A weighted-averaging unit provides a weighted-averaging signal to each of the filters and distributed gain is applied to the filter stages via a nonlinear function.